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[size=150][center]How do you do this problem:
"Find the sum of the arithmetic series."
"13+20+27+ ... +272"

Yours,
Highly Confused!!![/center][/size]
Guest Apr 25, 2012
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KatieeeBethhh16:

"13+20+27+ ... +272"



[input]sum ( 13+x*7, x=0..((272-13)/7) )[/input]

this is your series:
13+20+27+ ... +272
this is the series in reversed order:
272+265+258...+13

the sum of one element of the series and one element of the reversed series (at the same index) is always the same:
13+272=285
20+265=285
27+258=285
... = 285

you have (272-13)/7 +1 = 38 such terms.

then the sum of 2 times the arithmetic series is: 38 * 285 = 10830

the sum of the arithmetic series "13+20+27+ ... +272" is then: 10830/ 2 = 5415
admin  Apr 25, 2012

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